ccp, ccr: Simplify drivers to assume an AES-GCM IV length of 12.

While here, use crypto_read_iv() in a few more places in ccr(4) that I
missed previously.

Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D32104

(cherry picked from commit cb128893b92994456107d6ca722fdf6e5028eacc)